In various types of electronic systems, there is a need to retain some information across power cycles or during times of the interruption of power to the system. Often, in electronic data processing equipment, non-volatile memory is used for this purpose. The non-volatile memory may take the form of a non-volatile random access memory (NVRAM), which typically includes an erasable electrically programmable read only memory (EEPROM). The EEPROM is contained on an NVRAM chip, which also has a chip RAM used for transferring data to and from the EEPROM. The EEPROM is limited in the number of data storage operations which require a change of the data stored in the EEPROM.
An NVRAM chip contains a number of bits of storage, and the number of change operations for each bit is limited. An NVRAM chip may be specified as permitting 5,000 change operations at each bit location, for example.
In the case of an electrophotographic page printer, it is desirable to maintain a page count, indicative of the number of pages printed on the printer. This information can be of use in servicing the printer, for example.
A page counter for a printer will have a least significant digit which changes frequently, and the bits used in an NVRAM to maintain this digit change quickly. Therefore, in the case of a 5,000 change life for an NVRAM storage location, the implementation of a page counter capable of counting hundreds of thousands of pages is not directly possible.
One solution to this count limitation problem would be to employ NVRAM capable of a greater number of change operations, but such NVRAM is not readily and affordably available. Another solution would be to change the NVRAM chip at the end of its rated lifetime, but this is impractical due to the cost and inconvenience of making such changes.
Another technique would be to employ an NVRAM having many times the number of storage locations which are needed to store the data to be saved. Then, in the case of a counter for example, as bit storage locations reach their rated limit of change operations, the system employing the NVRAM can move data to new storage locations in the NVRAM. This approach is implemented in an electronic odometer described in U.S. Pat. No. 4,803,707.
The employment of this solution to the NVRAM life problem requires an application in which the data of concern are the less significant digits of a counter of some type so that the circuitry employing the NVRAM inherently can determine the number of changes to the less significant digits based on the count in the more significant digits. This solution also ignores the possibilities of NVRAM storage locations enduring for greater or fewer data change operations than that for which they are rated.
It is the general aim of the invention to provide an improved non-volatile memory management system in systems of the foregoing type which overcomes the stated disadvantages.
As will be described subsequently with regard to an exemplary embodiment, the invention will find advantageous use in electrophotographic page printers in which printer information as well as a count of printed pages are maintained in non-volatile memory.